I’ve been using Freshbooks(disclaimer: I get a commission if you sign up with them) since 2007 as my online accounting and invoicing service and in that time I’ve seen them change from a very basic online invoicing service to a full-service, cloud-based accounting system that includes projects, time tracking, estimates, reporting and an API that you can use to program your own customer support system. I’ve used Freshbooks to create estimates for website work, manage time in projects and invoice for completed work, hosting expenses and even billing for monthly rent on rental properties. It is efficient, simple and easy to use for customers receiving estimates and invoices.